1. Field of the Invention
This invention relates generally to making small openings or slots in shadow masks for color television tubes as well as other articles, more specifically, this is an etching process for producing accurate holes or openings in a continuous sheet of material in which the openings have a dimension smaller than the thickness of the material.
2. Description of the Prior Art
In a colored television picture tube, a shadow mask or aperture mask is located between the electron guns at the rear of the tube and the phosphor coated face plate at the face of the tube. Electron beams pass through tiny openings or apertures in the shadow mask and impinge upon a suitable color producing phosphor dot on the face plate. Located in line with the openings of the shadow mask are three phosphor dots, a triad, one dot for each of the three primary colors. During operation of the picture tube the shadow mask openings are used as a guide for the electron beams. Thus, one of the uses of the present invention is in a shadow mask for a television tube. Another application of the present process is in the manufacture of small slits or guides for use in reticles or the like in which slight openings or apertures are required to have the minimum dimension less than the thickness of the material.
The problem in making small holes or apertures in thicker materials is that it is impossible to accurately etch an opening which has a minimum dimension less than the thickness of the material. That is, the conventional etching from both sides produces over-etching as well as irregular etching. The over-etching is produced by the lateral etching of the material that inherently accompanies etching perpendicular to the surface of a material. Consequently, the process of accurately making holes having a minimum dimension less than the thickness of the material is a difficult task. The task is even more difficult when applied to a continuous production of etched material. Thus, the present invention comprises a process for continuously and accurately etching openings which have a minimum dimension less than the thickness of the material being etched. More specifically, the present invention comprises a process of continuously forming accurate holes which have a minimum dimension that is on the order of 40% of the thickness of the metal.
At the heart of the present invention is a reusable etchant resistant shield that covers the layer of etchant resist and one side of the web material during the initial phase of the etching process. During a later phase of the etching process the reusable shield is removed and the web material is etched from both sides.